2004-01-2345 Use of a Thermal Manikin to Evaluate Human Thermoregulatory Responses in Transient, Non-, Thermal Environments

Robert B. Farrington, Ph.D., P.E. John P. Rugh, M.S.M.E. Desikan Bharathan, Ph.D., P.E. National Renewable Energy

Rick Burke, B.S.M.E. Measurement Technology Northwest

Copyright © 2004 SAE International

ABSTRACT rejected by sweating, the body’s core temperature will increase, which will reduce response time, decrease People who wear protective that inhibit mental capacity and accuracy, and have potentially evaporation of sweat can experience reduced adverse health effects without personal cooling systems. productivity and even health risks when their bodies Additionally, the accumulation of sweat contributes to cannot cool themselves. This paper describes a new personal discomfort and premature aging of the garment sweating manikin and a numerical model of the human ensemble. thermoregulatory system that evaluates the thermal response of an individual to transient, non-uniform Accurately modeling human heat loss in a transient, thermal environments. The physiological model of the non-uniform thermal environment is difficult. Factors that human thermoregulatory system controls a thermal contribute to the complexity include: manikin, resulting in surface temperature distributions representative of the human body. For example, surface • Thermal radiation view factors. temperatures of the extremities are cooler than those of • Thermal radiation, convection, and conduction the torso and head. The manikin contains batteries, a between layers. water reservoir, and wireless communications and • Thermal and moisture capacitance of clothing. controls that enable it to operate as long as 2 hours • Clothing to skin contact area. without external connections. The manikin has 120 • Clothing to skin thermal resistance. separately controlled heating and sweating zones that • Clothing fit, including microvolumes. result in high resolution for surface temperature, heat • Non-uniform thermal properties of clothing flux, and sweating control. The physiological finite ensembles. element model uses approximately 40,000 solid thermal • Non-uniform, transient velocity field around the and blood network elements to represent the human body. body. The manikin and physiological model demonstrate Modeling evaporation of sweat. their value in evaluating the thermoregulatory response • of a person in a protective uniform. They can also be A thermal manikin and a finite element model of the used to evaluate the effectiveness of personal cooling human thermoregulatory system have been developed systems. to simulate the thermal response of a person in a transient, non-uniform environment with multiple heat INTRODUCTION transfer modes: thermal radiation, natural and forced convection, conduction, and evaporation. A third An objective of NREL’s manikin and model system is to computational tool predicts transient thermal sensation predict the effect of thermal environments on human and comfort, locally and globally. thermal physiology. The impermeable garment design of a protective diminishes the body's evaporative The thermal manikin, shown in Figure 1, is a cooling capacity, sometimes severely. If heat cannot be sophisticated sensor with 120 independently controlled be specified separately for each zone.

The manikin can evaluate the thermoregulatory response of a person who is wearing (1) a moisture- impermeable suit used by first-responder personnel such as Hazmat, (2) a flight suit, and (3) a battle suit (Figure 2). Other applications include chemical- warfare , extravehicular activity (EVA) suits, and personal cooling systems used with such suits.

The manikin measures the heat loss in a thermal environment and sends the heat flux from each zone to the physiological model. The physiological model predicts the body’s response to the environment, determines 120 zone skin temperatures, sweat rates, and breathing rates, and transmits the data to the manikin. The psychological comfort model uses temperature data from the physiological model to predict the local and global thermal comfort as a function of local skin and core temperatures and their rates of change [Zhang et al. 2003]. Figure 1. Thermal Manikin, ADAM THERMAL MANIKIN ADAM zones. Using this manikin as a sensor integrates the complex clothing and environmental heat exchange ADAM (ADvanced Automotive Manikin) was developed factors into local heat loss measurements from the skin to evaluate transient, non-uniform thermal environments surface. in automobiles. He is a sophisticated surface sensor that interacts with his environment. He not only responds to The physiological model can regulate the manikin for thermal inputs such as radiation and convection, but human-realistic spatial and temporal thermal response. also affects the environmental flow field and temperature The manikin can also be controlled with traditional field. The manikin was designed with the following regulation methods: constant skin temperature or general capabilities and characteristics: constant heat flux that corresponds to metabolic activity level. Setpoints are individually adjustable for each of • Detailed spatial and rapid temporal control of the 120 zone controllers. Volumetric sweating rates can surface heat output and sweating rate.

Figure 2. Personal Protective Equipment (Class B Hazmat, Flight Suit, Battle Dress Uniform with Gas Mask) • Surface temperature response time that measures heat loss into the manikin interior from that approximates human skin. zone. • Realistic and uniform sweating. • Human-like geometry and weight with prosthetic The manikin’s skeleton (Figure 4) is composed of joints to simulate the human range of motion. laminated carbon fiber, which supports its structure, • Breathing with inflow of ambient air and outflow houses all internal components, and provides mounting of warm, humid air at realistic human respiration locations for surface zones. The joints connect the rates. skeleton parts to give the manikin a human-like form. • Complete self-containment, including battery The adjustable friction joints are pre-tensioned so it can power, wireless data transfer, and internal sweat be posed in specific human positions. The wiring reservoir for at least 2 h of use with no external connections. • Rugged, durable, low-maintenance construction.

The geometry of the manikin was designed to match the 50th percentile American male. The manikin is approximately 175 cm tall. A Non-Uniform Rational B- Splines digital model of the human body was reshaped in CAD to comply with the 50th percentile target and allow the manikin to be manufactured with digital methods. He weighs approximately 61 kg.

The manikin’s fundamental components are the 126 individual surface segments, each with a typical surface area of 120 cm2. Each segment (Figure 3) is a stand alone device with integrated heating, temperature sensing, sweat distribution and dispensing, heat flux gauge, and a local controller to manage the closed loop operation of the zone. The sweating surface is all-metal

Figure 4. Manikin Skeleton System

harness and sweat tubes pass through the joints.

The manikin needs no external cabling. It uses the internal battery power pack (four internal NiMH battery modules in the torso and thighs) and a wireless communication system (Figure 5). The wireless communication system transfers data via 900 MHz spread spectrum transceivers. For applications that do not require wireless operation, the system can be plugged into an external power supply and communication port for continuous operation and battery charging. Self-contained operation is critical for EVA and Figure 3. Manikin Zone personal protective suits that are completely sealed from the ambient environment. construction optimized for thermal uniformity and response speed. Variable porosity within the surface The zone emissivity of 0.5 is lower than skin emissivity provides lateral sweat distribution and flow regulation of 0.95. This difference reduces the heat loss to the across the zone. Distributed resistance wire provides environment and is accounted for in the physiological uniform heating across the zone surface. Six segments model. are controlled in pairs, and result in 120 separately controlled zones. The single zone controller, including When the manikin is standing, two lower back zones flow control, is mounted directly on the back of a recess into its interior compared with a sitting position as segment [Burke et al. 2003]. The zone heat flux gauge shown in Figure 6. The physiological model accounts for permit inhalation and exhalation at a rate of 5 L/min. The breathing system can also permit continuous high levels of exhalation at 15 L/min. Humid, warm air is critical to evaluate sealed suits as breathing leads to moisture accumulation and heat gain into the enclosed environment.

PHYSIOLOGICAL MODEL OF THE HUMAN THERMOREGULATORY SYSTEM

The NREL Human Thermal Physiological Model, a three-dimensional transient finite element model, contains a detailed simulation of human internal thermal physiological systems and thermoregulatory responses. The model consists of a human tissue system and a thermoregulatory system. The thermoregulatory system controls physiological responses, such as vasomotor control, sweating, and shivering. The human tissue system represents the human body, including the physiological and thermal properties of the tissues. The model was developed with the commercially available finite element software ANSYS. This software can compute heat flow by conduction, convection, and mass Figure 5. Battery Pack Radio Water Storage transport of the fluid, which makes it practical for active and inactive zones, depending on the manikin’s simulating human heat transfer. posture. Human thermal response to an environment consists of The skin temperature of each zone is determined by an convection within the circulatory and respiratory array of thermistors, typically four, on each zone. A heat systems, and conduction within the tissues. The arms flux gauge integrated onto the internal surface of each and legs consist of bone, muscle, fat, and skin. There zone measures heat transfer between the surface zones are additional lung, abdominal, and brain tissues in the and the internal body cavity of the manikin. torso and head zones. The model calculates the conduction heat transfer based on the temperature Testing of the assembled manikin is underway to gradients between the tissue nodes. calibrate regional heat loss and sweat dispensing rates. Following calibration, the manikin will be correlated with Circulation heat transfer is modeled with a right-angled other manikins and human subject data for steady-state network of pipe elements within each body zone. The heat loss. diameter of the pipes decreases from the center of each zone outward toward the skin and extremities. The flow The breathing system will be installed in 2004. It will in the pipes is modeled as Poiseuille flow, and a

Figure 6. Recessed Manikin Zones convection coefficient is solved at each node in the pipe network. The diameters of the pipes in the skin layer can constrict or dilate depending on temperature. The equations that control vasoconstriction/dilation are based on medical experiments [Smith 1991].

The human thermoregulatory system is modeled with vasoconstriction/dilation, sweating, shivering, and metabolic changes. The vasoconstriction/dilation response varies with skin and core temperatures, and with each body zone, because of the diameters of the pipes. The sweating response is a function of skin and core temperatures, and the number of sweat glands in each zone. The degree of shivering depends on skin and core temperatures, and the amount of muscle in each zone. The cardiac output or flow through the pipe network is a function of the metabolic rate and skin and core temperatures [Smith 1991]. Figure 7. Hand Skin Temperature Distribution

The physiological model was generated in sections with Figure 8 shows the temperature distribution over the ANSYS. The sections consist of hand, lower arm, upper body as well as a cross-sectional view of the torso. The arm, foot, lower leg, and thigh, one each for the left and abdomen is warmer because of internal heat generation right sides. The body is developed as a torso together and relative isolation from the environment. The lung with neck and head. The limbs consist of bone, muscle, area remains cool because of breath flow. The brain fat, and skin. Each surrounds the previous layer. Special mass reaches a moderate temperature between the two. tissues for abdomen, lungs, and brain are introduced in the torso and head volumes. Each part is generated Given a set of heat flux boundary conditions on the skin, individually and populated with arteries and veins. The the model currently requires about 2 min to converge at primary blood vessels join via capillaries placed adjacent the steady-state temperature distribution. We expect to to the skin layer. The blood vessel diameters are sized reduce this time to 1 min by streamlining the model and to allow blood to flow to each body part at an overall eliminating the large amount of input/output that occurs nominal pressure difference of 70 mmHg between the during a normal ANSYS run. blood supply and return. The tissues are modeled with ANSYS Solid70 elements, and the blood flow pipes use Fluid116 elements. Tissue properties are taken from tables provided by Gordon et al. [1976]. The overall masses and mass distribution for each part in the model compare favorably to those of a human. Deviations are nominally less than 5%.

Each body part is connected to its adjacent part with veins and arteries. In the limbs, the tissues are not connected between parts. In the torso, which is modeled as an integral part, all tissues are connected. An additional pipe network to simulate airflow through the trachea and lungs is included in the torso.

The overall model consists of approximately 40,000 nodes and elements. Because the model is very detailed, we can see a fairly complete picture of temperature distribution. An example for the hand is shown in Figure 7. For this simulation, blood flows into Figure 8. Body Core Temperature Distribution the supply arteries at 1380 cc/h at 37°C. The hand’s muscle and skin tissues generate heat at 750 W/m3 and 3 In principle, the physiological model can operate 1005 W/m , respectively. The hand is exposed to a independently of the manikin. However, detailed lower temperature environment that is applied as a heat 2 knowledge of the transient, non-uniform, thermal loss of 100 W/m on its exposed surfaces. The resulting environment is required. For example, all the view temperature distribution on the external surfaces is factors for thermal radiation must be specified, local shown in Figure 7. The tip of the smallest finger attains evaporation rates determined, clothing properties the lowest temperature; the palm remains warm. (including fit, thermal resistance, thermal and moisture capacitance, microvolumes, etc.) specified, a detailed transient flow field calculated, etc. With the physiological The skin heat transfer rates are sent to the physiological model capabilities, the manikin responds to and interacts model, which computes the skin and internal with the thermal/fluid environment. temperature distribution and surface sweat rates. This information is then sent back to the manikin, which HUMAN THERMAL COMFORT EMPIRICAL generates the prescribed skin temperatures, surface MODEL sweat rates, and breathing rates. This loop continues to provide a transient measurement tool. The University of California, Berkeley, performed 109 human subject tests (Figure 9) in its Controlled An objective of the manikin and model system is to Environmental Chamber under a range of steady-state predict the effect of thermal environments on human and transient thermal conditions to explore the thermal physiology. The model can specify human relationship between local thermal conditions and metabolic rates of heat generation. In a protective suit, perception of local and overall thermal comfort. Core where sweat will not evaporate, the skin temperature will and local skin temperature data and subjective thermal increase. Because the generated metabolic heat cannot perception data were obtained via a simple form. These be rejected, the body’s core temperature will increase. data were used to develop a predictive model of thermal This reduces response time, decreases accuracy, and comfort perception [Zhang et al. 2003]. Details of the may have adverse health effects. subject testing and analysis are available in Zhang [2004]. PRELIMINARY TEST RESULTS

The manikin skin temperatures should decrease along the extremities as they do for a human. Figure 10 shows an infrared image that compares the manikin surface temperature with the skin temperatures for a human subject. The variations in surface temperature are similar for the manikin and the human subject.

The importance of skin sweating is shown in Figure 11. If a non-sweating manikin is used, there is no skin surface cooling effect and the resulting error can be high.

Preliminary measurements for a Class A Hazmat suit are shown in Figure 12. The tests were conducted with one arm sealed in a Class A suit sleeve and the other arm bare and exposed to room temperature. Both arms had sweating, but the sweat could not evaporate for the enclosed arm. The resulting increase in skin temperature where the sweat could not evaporate is Figure 9. Thermal Comfort Human Subject Testing significant. The subject sample size was somewhat limited, and did not include a wide variety of ages, weights, and body compositions. The application of the thermal comfort model may be limited in evaluating personal protective clothing. Accumulation of sweat on the skin increases skin-wettedness, which, along with the mechanical sensation of such suits, may limit the usefulness of thermal comfort predictions.

MANIKIN AND MODEL INTEGRATION

The integrated system consists of the thermal manikin and the physiological model. The manikin provides a simulated body positioned in a complex thermal environment to measure the transient thermal response with extremely high spatial resolution. The finite element model provides the manikin with a control algorithm that represents human thermal response.

The manikin is essentially a surface sensor that measures the rate of heat loss at each surface zone. Figure 10. Infrared Image of Manikin and Human

Figure 11. Impact of Non-Sweating Skin Figure 12. Hazmat Class A, Arm Skin Temperature

CONCLUSION ACKNOWLEDGMENTS

A next-generation manikin has been developed to The authors gratefully acknowledge the support of the respond to and interact with a transient, non-uniform U.S. Department of Energy in developing, fabricating, thermal environment. This self-contained manikin has and testing the thermal manikin and physiological model. high spatial resolution, a reasonable response time, and DOE’s Office of FreedomCAR and Vehicle Technologies realistic and uniform sweating rates. A physiological (OFCVT) supported this work. The authors appreciate model was developed that mimics human thermal the support of Roland Gravel, DOE Program Manager; responses. Although not discussed in detail here, a Terry Penney, NREL’s OFCVT Technology Manager; thermal comfort model has also been developed. and Barbara Goodman, Director of the Center for Transportation Technologies and Systems. Preliminary testing with infrared imaging has demonstrated similar temperatures between the manikin REFERENCES surface and human subject skin. Testing under thermal radiation demonstrates the need to include sweating in 1. Burke, R.; Rugh, J.; and Farrington R. (2003) ADAM manikin testing, in cases where sweat evaporates and – the Advanced Automotive Manikin, 5th cools the skin. International Meeting on Thermal Manikins and Modeling, Strasbourg, France. In cases where evaporation of sweat is impeded, skin 2. Gordon, R. G.; Roemer, R. B.; and Horvath, S. M. and body core temperatures increased. (1976) A Mathematical Model of the Human Temperature Regulatory System – Transient Cold The manikin and physiological model are new tools that Exposure Response. IEEE Transactions on will be useful in predicting the thermoregulatory Biomedical Engineering 23, no. 6: 434-444. response of a person in a protective uniform. They can 3. Smith, C. E. (1991) A Transient, Three-Dimensional also evaluate the effectiveness of personal cooling Model of the Human Thermal System. PhD Thesis, systems used with these types of suits. Those who Kansas State University. design and evaluate protective clothing, such as that used by the military, NASA, and first-responders, will 4. Zhang, H.; Huizenga, C.; and Arens E. (2003) find these new tools useful. Thermal Sensation and Comfort in a Transient Non- Uniform Thermal Environment, 5th International Meeting on Thermal Manikins and Modeling, Strasbourg, France. 5. Zhang, H.; (2004) Human Thermal Sensation and Comfort in Transient and Non-Uniform Thermal Environments, Ph.D. Thesis, U.C. Berkeley.

CONTACTS

Robert B. Farrington, Ph.D., P.E., manages the Advanced Vehicle Systems (AVS) Group for the National Renewable Energy Laboratory’s (NREL) Center for Transportation Technologies and Systems. The AVS Group works on advanced power electronics, vehicle ancillary loads, heavy hybrid vehicles, vehicle modeling, and energy storage systems. He can be reached at: [email protected]

John Rugh, M.S.M.E., leads the vehicle ancillary loads reduction task at NREL. The objective of the project is to reduce fuel used for vehicle ancillary loads by reducing thermal loads, improving delivery efficiency, and increasing equipment efficiency including using engine waste heat to provide cabin cooling. He can be reached at: [email protected]

Desikan Bharathan, Ph.D., P.E., is a heat transfer and fluid dynamics expert. He has contributed to ocean thermal energy conversion systems, geothermal systems, building systems, and vehicle systems. He can be reached at: [email protected]

Richard A. Burke, B.S.M.E., is Vice President at MTNW in charge of the biophysical instrumentation department. Since joining the company in 1989, he has played a key role in the development of a growing line of high- accuracy biophysical test equipment. His broad expertise includes manikin fabrication methods, biophysical testing, and imbedded control systems. He can be reached at: [email protected]

ADDITIONAL SOURCES

NREL: http://www.nrel.gov

Vehicle Ancillary Loads: http://www.ott.doe.gov/coolcar/

MTNW: http://www.mtnw-usa.com/aboutmtnw/